Abstract

The catalytic activity of an enzyme is profoundly affected by its ionisation state, whether it is dissolved in aqueous solution or suspended in low water organic media. In aqueous solution, counter ions can freely move around in solution. Since they are not closely associated with opposite charges, their identity does not effect the protonation state of the enzyme and so pH alone governs the protonation state. When a biocatalyst is suspended in low water organic solvent, the situation is more complex. In this case, counter ions are in closer contact with the opposite charges on the enzyme because of the lower dielectric constant of the medium. Thus, protonation of ionisable groups on the enzyme will be controlled by the type and availability of these ions as well as hydrogen ions. Changes in ionisation state of the protein can therefore be described by 2 equilibria which can, in theory, be controlled independently.